1. Field of the Invention
The invention is related to the production of group III nitride wafers using the ammonothermal method.
2. Further Information on Group III-Nitride Materials and Manner of Making
Gallium nitride (GaN) and its related group III alloys are the key material for various opto-electronic and electronic devices such as light emitting diodes (LEDs), laser diodes (LDs), microwave power transistors, and solar-blind photo detectors. Currently LEDs are widely used in cell phones, indicators, displays, and LDs are used in data storage discs. However, a majority of these devices are grown epitaxially on heterogeneous substrates, such as sapphire and silicon carbide, since GaN wafers are extremely expensive compared to these heteroepitaxial substrates. The heteroepitaxial growth of group III nitride causes highly defected or even cracked films, which hinders the realization of high-end optical and electronic devices, such as high-brightness LEDs for general lighting or high-power microwave transistors.
To solve fundamental problems caused by heteroepitaxy, it is useful to utilize single crystalline group III nitride wafers sliced from bulk group III nitride crystal ingots. For a majority of devices, single crystalline GaN wafers are favorable because it is relatively easy to control the conductivity of the wafer, and GaN wafer will provide smallest lattice/thermal mismatch with device layers. However, due to high melting point and high nitrogen vapor pressure at high temperature, it has been difficult to grow group III nitride crystal ingots. Growth methods using molten Ga, such as high-pressure high-temperature synthesis ([1] S. Porowski, MRS Internet Journal of Nitride Semiconductor, Res. 4S1, (1999) G1.3; [2] T. Inoue, Y. Seki, O. Oda, S. Kurai, Y. Yamada, and T. Taguchi, Phys. Stat. Sol. (b), 223 (2001) p. 15) and sodium flux ([3] M. Aoki, H. Yamane, M. Shimada, S. Sarayama, and F. J. DiSalvo, J. Cryst. Growth 242 (2002) p. 70; [4] T. Iwahashi, F. Kawamura, M. Morishita, Y. Kai, M. Yoshimura, Y. Mori, and T. Sasaki, J. Cryst Growth 253 (2003) p. 1), have been proposed to grow GaN crystals, nevertheless the crystal shape grown in molten Ga favors thin platelet formation because molten Ga has low solubility of nitrogen and a low diffusion coefficient of nitrogen.
An ammonothermal method, which is a solution growth method using high-pressure ammonia as a solvent, has demonstrated successful growth of bulk GaN ingots ([5] T. Hashimoto, F. Wu, J. S. Speck, S. Nakamura, Jpn. J. Appl. Phys. 46 (2007) L889). This newer technique called ammonothermal growth has a potential for growing large GaN crystal ingots, because high-pressure ammonia used as a fluid medium has high solubility of source materials, such as GaN polycrystals or metallic Ga, and has high transport speed of dissolved precursors. However, state-of-the-art ammonothermal method ([6] R. Dwiliński, R. Doradziński, J. Garczyński, L. Sierzputowski, Y. Kanbara, U.S. Pat. No. 6,656,615; [7] K. Fujito, T. Hashimoto, S. Nakamura, International Patent Application No. PCT/US2005/024239, WO07008198; [8] T. Hashimoto, M. Saito, S. Nakamura, International Patent Application No. PCT/US2007/008743, WO07117689; See also U520070234946, U.S. application Ser. No. 11/784,339 filed Apr. 6, 2007), can only produce brownish crystals. This coloration is mainly attributed to impurities. In particular, oxygen, carbon and alkali metal concentration of the sliced wafers from GaN ingots is extremely high. The brownish wafer shows large optical absorption, which deteriorates the efficiency of light emitting devices grown on such wafers.
Each of the references above is incorporated by reference in its entirety as if put forth in full herein, and particularly with respect to description of methods of making using ammonothermal methods and using these gallium nitride substrates.